Transcript Lecture

Saturday Morning Physics
Feb. 18, 2012
Brian Utter
The Race to the Bottom
By the mid-1800’s, the
temperature of absolute zero
was accurately predicted
1883 – Wrobleski liquefies
nitrogen (77 K)
1898 – Dewar liquefies
hydrogen (20.28 K)
Sir James Dewar
Scottish chemist
Royal Institute of London
1898 – Dewar solidifies
hydrogen (14.01 K)
1908 – Onnes liquefies
helium (4.2 K)
Heike Kamerlingh-Onnes
Dutch physicist
University of Leiden
(1853 – 1926)
(Non-super) Conductivity
Each electrons moves fast (around
1,000,000 m/s)…
… BUT, they are constantly bombarding
the atoms in the material.
Not quite right, but like “microscopic Plinko”
In the end, electrons slowly “drift” along
at about 1 meter per hour!!
They lose a lot of energy in collisions,
which is lost as heat.
This is called electrical resistance.
An Experiment
Onnes was interested in how the
electrical properties of matter were
affected by temperature. What happens
with a “normal” conductor, like copper,
if you measure the resistance as the
temperature is decreased?
An Experiment
Onnes was interested in how the
electrical properties of matter were
affected by temperature. What happens
with a “normal” conductor, like copper,
if you measure the resistance as the
temperature is decreased?
At low temperature, the
resistance gets small, but
is limited by impurities.
Another Experiment
current sent
through resistor
voltage across resistor
(proportional to
(larger voltage = lower
The Original Notebook
“Kwik nagenoeg nul” translated as “Mercury practically zero”
meaning mercury’s resistance was practically zero at 3K.
Sudden drop to zero
resistance below
critical temperature.
The Nobel Prize in Physics 1913 was awarded to Onnes "for
his investigations on the properties of matter at low
temperatures which led to the production of liquid helium".
Without realizing it, they also observed the superfluid transition -- two different
quantum transitions seen for the first time in one lab on the same day!
Another Experiment
Superconductors exhibit “perfect conduction.”
But, there’s more weirdness – it’s not just a perfect conductor.
There are other behaviors that can’t be explained just as a
conductor with zero resistance.
It also exhibits the Meissner effect, discovered by German
physicists Walther Meissner and Robert Ochsenfeld twenty
years later in 1933.
Meissner Effect
Expulsion of magnetic fields
normal conductor
An explanation, 5 dacades later:
BCS Theory (1957)
John Robert Schrieffer , John Bardeen, and Leon Cooper who
developed the BCS Theory of superconductivity, for which they were
awarded the Nobel Prize in Physics in 1972 ("for their jointly
developed theory of superconductivity, usually called the BCS-theory”).
Ingredient #1: Cooper Pairs
 Electron #1 deforms lattice of positive ions
 Electron #2 sees region of slightly higher positive charge
 Electron #2 is attracted to this slightly denser region
 and is therefore effectively attracted towards the first electron!!
Cooper pairs are effective attractions between two
electrons due to interaction with the solid lattice.
Ingredient #2: Bose-Einstein Condensate
Due to quantum mechanics, these electrons (which normally can’t
occupy the same place) can pile up and exist in sync with each other.
Electrons travel together as waves, like light shining through
the conductor, without bouncing off the atoms! The Cooper
pairs are a superfluid – no dissipation!
Ok, so I got my superconductor. Cool.
Now what?
Josephson and SQUIDS
In 1956, British physicist Brian
Josephson predicted the
behavior of current across a thin
insulator between two
superconductors (quantum
tunneling of Cooper pairs).
Used to make SQUIDs
(Superconducting QUantum
Interference Device) which can make
sensitive measurements of magnetic
fields. Fields as low as 10–18 T
(100,000,000,000,000 times weaker
than the Earth’s gravitational field!)
Josephson and SQUIDS
Leo Esaki, Ivar Giaever, and Brian D. Josephson (1973), "for their
experimental discoveries regarding tunneling phenomena in
semiconductors and superconductors, respectively," and "for his
theoretical predictions of the properties of a supercurrent through a tunnel
barrier, in particular those phenomena which are generally known as the
Josephson effects"
Superconducting Magnets
In 1962, the first commercial
superconducting wire, a niobiumtitanium alloy, was developed by
researchers at Westinghouse,
allowing the construction of the first
practical superconducting magnets.
(electromagnet == using a current to
create a magnetic field)
Superconducting Magnets
Superconductors can maintain a current
with no applied voltage. Experiments
show that currents in superconducting
coils can persist for years without any
degradation and a predicted lifetime of
at least 100,000 years! Theoretical
estimates for the lifetime of a persistent
current can exceed the estimated
lifetime of the universe!!
e.g. used in MRI machines.
High Tc Superconductors
Before 1980, it was believed that 30 K was the highest
possible temperature for a superconductor… until two
researchers at Bell Labs discovered “YBCO” (a ceramic)
with a critical temperature of 90K!
High Tc Superconductors
 The “holy grail” is a room temperature superconductor.
High Tc Superconductors
The Nobel Prize in Physics 1987 was awarded jointly to J. Georg
Bednorz and K. Alexander Müller "for their important breakthrough in the discovery of superconductivity in ceramic materials"
Theoretical Understanding
Alexei A. Abrikosov, Vitaly L. Ginzburg, and Anthony J. Leggett
(2003), "for pioneering contributions to the theory of
superconductors and superfluids."
Power Transmission
Holbrook Superconductor project, the world’s first transmission power cable
transmitting waves of electricity from the grid to a substation that feeds homes
in Long Island. This project includes 99 miles of 138 kV high-temperature
superconductor lines that are cooled with liquid nitrogen. (July 2008)
MagLev Trains
MagLev Trains
The highest recorded speed of a maglev train is 581 km/h (361 mph),
achieved in Japan by the CJR's MLX01 superconducting maglev in 2003
The first 100 years include strange
behavior, unexpected explanations,
and a variety of practical
applications. A room temperature
superconductor would open up a
new world of uses. Is this
impossible or the next revolution?